Mac Pro 2013 Quad Core Upgrade Question

I have a Mac Pro 2013 Quad Core 3.7 with 64 gig of ram and a 500 gig SSD and was wondering where I can send my unit to have it upgraded to a 8 or 12 core processor at a reasonable cost? I have read so many different forums about some processors being better then others and some upgrade kits having issues of overheating and so on. I just want to a service place I can send my unit to and have a quailty upgrade properly performed.

I know my memory will probably have to be upgraded after the processor change, but I expected that. Any recommendation would be greatly appreciated. Any idea of the cost I would be looking it for a complete upgrade including the memory? Can i still use the current SSD hard drive I have installed?

Thanks

Ron